The present invention relates to a device which makes it is possible to arrange for the measurement of external pressure in a wristop instrument, such as a sports computer, a diving computer, a wristwatch, or the like.
Sports computers and other light electronic devices such as GPS devices and similar, must, especially when they are intended to be worn on the wrist, be constructed in such a way that they meet the watertightness standards of the clock and watch sector. Such standards are, for example ISO 2281 and ISO 6425. The standards require products to remain watertight under water at a pressure according to a set depth. Wristop computers are usually specified as being watertight to 100 m. The test pressure is then 10 bar. Other watertightness classes used are 30 m, 70 m, and 200 m. Though the pressure class required in each case is defined according to the purpose of the device, in this connection it is important for the device to withstand the pressure under water. It may be sufficient for a device intended for light outdoor use to withstand a short immersion in water, for example, when swimming or otherwise moving in water, or for instance when washing dishes. In devices intended for more demanding use, a higher watertightness class may be necessary, only on account of the requirement for greater durability. Devices intended for diving use will of course have greater watertightness requirements, while aviation use will also place its own demands on the construction of such a device.
One way to implement pressure measurement is to use a separate pressure sensor, for which a through-hole is arranged in the body of the device. Such a through-hole is, however, expensive and takes up much space. If the device itself must be watertight, the pressure sensor can be put outside the watertight case through a sealed hole. In order to work, this construction demands complicated mechanical protection for the pressure sensor and constructions that make the case structure of the device watertight. However, these also create a thermal bridge, on which the internal moisture of the device can condense if the temperature of the bridge drops below the dewpoint of the gas inside the device.
Publication U.S. Pat. No. 6,754,137 discloses a construction, in which a piezoelectric pressure sensor is located in a wristwatch. The pressure sensor is enclosed in a space, which is connected to the environment, and which is isolated from the watertight internal space of the watch. In the solution of publication U.S. Pat. No. 4,783,772, a pressure sensor is located in a protrusion formed on the side of the case of a watch, in which there is space for the sensor and connections for transmitting a signal to the circuit board of the watch. Publication EP 1 024 034 discloses a pressure-sensor system, in which a gas-permeable membrane protects the actual sensor. In publication U.S. Pat. No. 6,016,102, a gas-selective membrane is used to seal the case of the pressure sensor, in order to be able to use the sensor in the case to measure the pressure of a specific gas.
In devices worn on the wrist, the sensor should be located in the device in such a way that it does not increase the external dimension of the device, but on the other hand also so that it does not block the path to the sensor. A location on the upper surface of the device is limited by the fact that as much of the upper surface as possible is generally required for the display. The opening or path going to the sensor can be closed using a membrane that is watertight but gas-permeable. Such membranes include those made from stretched polytetrafluoroethylene (PTFE). Known commercial names of such membranes are GORETEX™ and SYMPATEX™. The protective membrane must be well ventilated and must not be exposed to severe mechanical stress. The ventilation of the membrane is important, because, if the membrane becomes waterlogged and covered with liquid, its gas-penetrability and thus its pressure-transmission ability will be substantially reduced.